This invention relates to apparatus and processes for electrolysis usage, and more specifically to apparatus and processes for electrolytically producing a metal product.
Prior art electrolytic cells for electrowinning metal products have commonly consisted of closely spaced anode and cathode plates immersed in electrolyte. The planar surfaces of the electrode plates are generally spaced close together to minimize resistance to the flow of current through the electrolyte between the plates. However, in the electrowinning process, the depositing or plating of product is never uniform across the plates and some minimum clearance is required to avoid a short circuit contact between the cathode and anode. In the production of particulate metal products as taught in U.S. Pat. No. 4,025,400, the plates must be spaced sufficiently far apart to allow for crystalline dendritic growth on the cathode and provide sufficient space to accommodate apparatus for removal of the product while the cell is in operation. In the aforementioned method of producing a crystalline product, a relatively high current density is required to achieve the desired loosely adhering dendritic crystalline growth on the electrode. This high current density, combined with high voltages between the electrodes, results in high resistance heating of the electrolyte with attendant inefficient utilization of power in electrowinning.
For the hydrometallurgical recovery of metals from ores or ore concentrates using a chloride system, as taught, for example, in U.S. Pat. No. 3,785,944, insufficient stirring of electrolyte causes chlorine gas to evolve. Chlorine gas is harmful to the anode material of the cell as well as the metallic and plastic parts associated with the cell. This results in high maintenance costs to repair any parts damaged by the corrosive atmosphere, since all adjacent cells must be shut down to allow access to a damaged part and to avoid electrical hazard to the maintenance personnel.
Additionally, the partial envelopment of the anode by very small gas bubbles in an electrolysis process results in increased cell voltage. This difference in cell voltage with and without the gas bubbles on the anode is called "polarization" voltage. Prior art cells are generally subject to polarization voltage and their overall cell voltage could be reduced by eliminating the gas bubbles on the anode.
Prior art cells embodying the invention of U.S. Pat. No. 4,025,400 for making particulate metal crystals employ a mechanical arm to sweep across the area adjacent to the surface of the cathode for the purpose of mechanically limiting the growth of crystalline dendrites by dislodging some of the crystalline product from the cathode. This dislodged product then falls to the bottom of the cell for recovery and, generally, later processing. These mechanical arms have proven to be somewhat unsatisfactory in long-term service because some of the crystals adhere to the cathode with the result that the cathode must be periodically removed and cleaned to remove the accumulated adhering dendrites. Other mechanical techniques, such as employing a vibrating or oscillating cathode as taught in Italian Pat. No. 1,400,758 to Piro et al., or means for periodically raising and dropping a cathode, as shown in British Pat. No. 13,499 to MacKay, have been suggested to remove crystalline growth from a cathode. These suggestions have not proved to be acceptable from a cost, maintenance or growth removal standpoint.
These and other limitations and disadvantages of the prior art are overcome by the present invention and an electrolytic cell and process is provided that minimizes cell voltage, power requirements and resistance heating, substantially eliminates anode polarization voltage, and in a preferred embodiment provides for more positive removal of crystalline product.